Petrology of the Sutherland Commonage melilitite intrusives
The petrology of the Sutherland Commonage olivine melilitite intrusives have been investigated using petrographic and chemical methods. The occurrence consists of a ring dyke which surrounds a centrally located sill complex. The rock of the ring dyke is a typical melilitite which consists of olivine in a groundmass of melilite, clinopyroxene, opaque spinel, nepheline and perovskite. The sill complex is a multiple intrusion and is comprised of a lower green melilitite and an overlying (and younger) grey melilitite. The green melilitite is deuterically altered and the original mineralogy is destroyed to a large extent. The grey melilitite contains autoliths of the green and is a fairly typical monticellitic melilitite in which phenocrysts of olivine are set in a groundmass of melilite, monticellite, opaque spine!, nepheline and perovskite. Microprobe analyses of clinopyroxenes indicate that they are aluminous titanian diopsides and salites which exhibit complex zonation patterns. They record magmatic conditions ranging from the intrusive stage to a final phase of magmatic evolution during which a vapour phase evolved after the majority of the groundmass minerals had crystallised. The chemistry of olivine phenocrysts suggests that the parent magma to the Commonage intrusives accumulated in a temperature-zoned reservoir at the base of the lithosphere. Large, unzoned olivine phenocrysts crystallised in this chamber. Subsequent rupture of the chamber and ascent of magma led to supercooling and the crystallisation of abundant, strongly zoned phenocrysts of smaller size. Olivine crystallisation continued until the magma reached crustal levels. It is inferred from the chemistry of chromites and magnetites that the magma in the ring dyke was more evolved than those in the sill complex and that very oxidising conditions prevailed in the grey melilitite during the crystallisation of magnetite in this intrusive type. The high fO₂ may have resulted from the degassing of CO₂ after intrusion. Major and trace elements have been analysed for in eleven whole rock samples and the ⁸⁷Sr/⁸⁶Sr ratio was determined for seven of the same samples. The results of the geochemical study suggest that the Commonage melilitites were derived by the melting of a recently metasomatised region of the asthenosphere, probably under the influence of an ocean-island-type hotspot situated in the lower mantle.
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